Presently, analog facsimile (fax) information is transmitted over the public switched telephone network (PSTN), where the connections are formed just like voice connections, by direct dialing (or tone keying) of a called (or receiving or destination) site from a sending (or originating) site. Typically, the entire connection route is internal to the PSTN. Upon establishment of a connection, the sending site sends signals representing facsimile image information to the receiving site.
Problems with this are: 1) for the PSTN customer, fax transfers to busy or intermittently faulty destinations can be difficult to complete, and transfers to remote destinations can be costly (especially transfers of fax images of bulky documents); 2) for PSTN carriers managing such transfers, revenues can be small in relation to the service being provided, taking into account repeated handling of calls to busy destinations.